The need for knowing the concentrations of various environmental pollutants in rivers and soils in a quest to preserve the environment is increasing and establishment of measurement technology for a very small quantity of pollutant has been demanded because there are some pollutants which are very toxic to humans even the quantity thereof is very small. There is a pollutant named dioxin which recently attracts public attention, and methods to use a gas chromatography mass spectrometer and an ELISA method (enzyme-linked immunosorbent assay) are known as a method to determine this dioxin. The gas chromatography mass analyzer can perform a trace analysis with a high degree of precision in an order of 10−12 g/ml. However, there are disadvantages in that the price of the instrument is extremely high, which results in a high analysis cost, and a long period of time is necessary for analysis. Though the ELISA method is more inexpensive in analyzer price, and in analysis cost, and faster in required time for analysis, compared with those in the gas chromatography mass analyzer, but a problem is a low analytical precision in the order of 10−9 g/ml.
Then, the present inventor pays attention to a crystal oscillator as a measuring instrument for pollutant such as dioxin or the like, from the fact that once a substance to be detected is attached to a crystal oscillator, the natural frequency of the crystal oscillator varies according to the amount of the attachment. On the other hand, there is a technology as a chemical sensing instrument using a crystal oscillator, which is described in Patent Document 1. The instrument includes a sampling circuit for outputting the absolute value of a difference frequency between the oscillation frequency of a sensor oscillator and the reference frequency generated by the reference oscillator, a frequency divider circuit dividing the difference frequency by a prescribed frequency divider ratio, a counter for counting the cycle of the frequency diver output using the cycle of the reference frequency as a clock, and a calculating device for determining the oscillation frequency of a sensor oscillator based on the counted cycle, and is for performing the identification of an adsorption gas. Since the chemical sensing instrument determines a difference frequency, it has an advantage of making the absolute value of a frequency to be measured small, and making it possible to perform measurement with a high resolution without enlarging the measurement range.
However, the technology in Patent Document 1 is designed to lower the clock frequency of the counter by using the frequency divider circuit. Accordingly, if the above-described difference frequency is high, the number of stages of the frequency divider circuit increases, which results in increase of the phase noise. Therefore, the above-described difference frequency cannot be made so high practically, which brings a problem of difficulty in assuring a high measurement precision. As a result, the range of application is limited, and it is difficult to apply the technology when detection of a very small quantity of material such as dioxin or the like with a high degree of precision is required. Furthermore, since it uses a counter, there is the problem of lengthy measuring time if a high resolution is required.
Patent Document 1
Japanese Patent Application Laid-open No. Hei 6-241972